1. Field of the Invention
The present invention relates to an image forming apparatus utilizing a control unit provided with program memories, for example an image forming apparatus utilizing, as said control unit, a one-chip microcomputer (hereinafter referred to as microcomputer).
2. Description of the Prior Art
The remarkable progress in electronic technologies in recent years has accomplished integration of electronic circuits and has provided highly advanced microcomputers which are now utilized for controlling various devices.
In the following there will be given an explanation on a conventional control unit utilizing a microcomputer for controlling an electrophotographic copying apparatus as shown in FIG. 1, of which functions are to be explained later.
In this connection reference is made to FIG. 2 showing a conventional control system for a copying apparatus, FIG. 3 showing a display switching subroutine, FIG. 4 showing the output timing wave forms of output terminals R1-R4 to be explained later, and FIG. 5 showing a procedure of parallel operations by a microcomputer of drum clock signal reading and display during copy cycle, said procedure being stored in an unrepresented memory in the microcomputer MCO.
In FIG. 2 there are shown a one-chip microcomputer MCO; a key input matrix KM; 7-segment display devices SET1, SET2, COPY1 and COPY2 composed for example of light-emitting diodes or liquid crystal display elements, wherein SET1 and SET2 are display devices for respectively indicating the first and second digit of the set number to be copied, and COPY1 and COPY2 are display devices for respectively indicating the first and second digit of the number of copies already made. There are also shown discretely settable and resettable output ports R, in which the port R1, in a set state thereof, i.e. when releasing a digital "1" signal, light the display device COPY2 and enables entry, into the ports K to be explained later, of the key input signal from the corresponding key "0", "1", "2" or "3" of the key input matrix KM. The actuated key is identified by the port K receiving said signal. The ports R2, R3 and R4 similarly light the display devices COPY1, SET2 and SET1 respectively and enable the entry of key input signals into said ports K. There are also shown an output port R5 for releasing a signal for lighting a jam indication lamp in case of a jamming in the copier, an output port R6 which is set to release a signal for reversing the optical system upon completion of optical scanning of an original, an output port R7 which is set at the start of said optical scanning to release a signal for advancing the optical system and lighting the exposure lamp, a paper feed signal output port R8 which is set, at the timing of starting paper feeding with a paper feed roller 28 continuously rotated after the start of copying operation, to activate a paper feed solenoid thereby lowering said paper feed roller 28 and thus initiating the paper feeding, and an output port R9 which is set at the turning on of the power supply to activate a main motor and a high-voltage transformer. The ports K are input ports for entering key inputs and allowing discrete check of the input signal. Also the ports X are input ports allowing discrete check of the input signal, wherein the port X1 receives a signal OHM indicating the arrival of the optical signal at a home position, while the port X2 receives clock pulse signals synchronized with the rotation of drum. The ports S release, from an unrepresented register in the microcomputer MCO, data of the first and second digits of the set number to be copied and of the first and second digits of the already copied number, as 4-bit parallel signals, to a display decoder DD which converts said data into 7-bit signals for display on said display devices SET1, SET2, COPY1 and COPY2.
FIG. 3 shows the display switching subroutine which functions, though detailed explanation for each step being omitted, to set the output ports R1 to R4 in succession thereby performing dynamic display and enabling input of key input signals from the key input matrix KM corresponding to said output ports R into the input port K. Thus, when the copy cycle is not in operation, the output signals from the output ports R1-R4 have a constant duty ratio as shown in the left-hand half of FIG. 4 as the program is only required to repeat the displays and the sensing of key entry signals.
On the other hand, even after the copy cycle is initiated, the dynamic display has to be continued though the entry of key inputs is no longer required. Also it becomes necessary to receive, from the input port X2, the drum clock pulses DCK generated mechanically, magnetically or optically and to count said pulses in parallel in order to trace the functioning position of the copier. FIG. 5 shows the flow chart of said operation, wherein the loop a corresponds to the duration of level "1" of drum clock pulses DCK shown in FIG. 6 and the loop b corresponds to the duration of level "0" of said drum clock pulses.
In the flow chart shown in FIG. 5, the program identifies, at the step 5-1, if the input port X2 receiving said drum clock pulses at a level "1", then executes the display switching subroutine if the port X2 is at level "1", proceeds at the trailing end of drum clock pulse DCK to the step 5-2 in which there is again identified if said input port is at a level "1", then executes the display switching subroutine if the port X2 is at the level "0", and proceeds to the step 5-3 at the leading end of the drum clock pulse DCK. In the step 5-3, 1 is subtracted from the data of the first digit of a pulse to be counted which is previously set and stored in a memory address A1 and the result of said subtraction is stored in said address A1. Said memory address A1 and other memory addresses A2 and A3 to be explained later store the numbers of pulses to be counted in a form of hexadecimal codes. In the succeeding step 5-4 the program identifies if the content of said memory address A1 has become 15, and, if not, returns to the step 5-1 to continue the pulse counting, or, if so, proceeds to the step 5-5 in which 1 is subtracted from the data of second digit of the pulse number to be counted which is previously set and stored in the memory address A2 and the result of subtraction is stored into said address A2. In the succeeding step 5-6 the program identifies if the content of said address A2 has become 15, and, if not, returns to the step 5-1. Similar identification is repeated also for the memory address A3 storing the third digit of the previously set pulse number to be counted until all the contents of said memory addresses A1, A2 and A3 become 15, when the output port R6 is set to release a signal for reversing the optical system.
Thus, if the drum clock pulse DCK is initiated after the resetting of port R1 and setting of port R2 in the display switching subroutine in the loop b, the program returns to the step 5-1 through the loop l unless the content of address A1 becomes equal to 15, and initiates the display switching subroutine to reset the port R2 and set the port R3. Since the time required for a single instruction of a microcomputer is constant, the output signals from the output ports R1 and R2 thus show significantly different duty ratios as shown in the right-hand half in FIG. 4, resulting in a difference in the durations of display by the display device COPY2 and COPY1 and thus in a flickering in the display. Also significant differences in the duty ratios for the ports R1 to R4 may hinder key input operation, disabling secure entry for example of a stop instruction during a copy cycle. Such drawback has been basically unavoidable as long as a microcomputer performs a sequential control.